The trick to fatherhood has a lot to do with the brain — and how close a dad gets to his kids. At least that's the message from a mounting pile of research into the neurological and hormonal cues that translate into fatherly nurturing. And what better time to keep that message in mind than on Father's Day?

"Mothers have an advantage, in that the hormones of pregnancy give them a head start and get them primed to be nurturing," said James Rilling, an anthropologist at Emory University who specializes in studying the neurological basis of social behavior. "In particular, when women give birth, there's a big surge of oxytocin, and oxytocin is also released during breastfeeding. Fathers don't have that."

It turns out that fathers get many of the same rushes that mothers do from parenthood — but the payoff depends on proximity and interaction. For example, researchers see the effect if the child sleeps with the parents, if the father recognizes and responds to the baby's cries, if Dad plays with the kids. When that proximity isn't present, the fatherhood effect isn't as strong.

"There seems to be some kind of fundamental social-neurobiological framework that comes into play when fathers interact with their kids," said Lee Gettler, an anthropologist at Notre Dame who worked on the prolactin study.

Why is it that the mothers and fathers come to the same hormonal response through different paths? "It may be that the most parsimonious way to engineer a paternal brain would be to take the circuitry that was already in place for maternal care, and maybe tweak that," Rilling said. "That might be the reason why there's some overlap there."

James Swain et al. / U. of Michigan

This functional magnetic resonance image shows areas of heightened brain activity when a father hears his own child's cries. Notable areas of activity include the frontal cortex, insula putamen, thalamus and superior temporal cortex.

Or it may merely be that when it comes to parenting, familiarity breeds fatherhood. University of Michigan psychiatrist James Swain has been analyzing a huge data set of MRI snapshots to see how maternal and paternal brains respond to the cries of their own babies and the children of strangers. He and his colleagues have found that brain activity patterns don't change as quickly for fathers as they do for mothers.

"I joke that this may be the physiological basis for why a father can roll over in bed when the baby's crying at 3 weeks," Swain told NBC News.

However, by the 4-month mark, "the fathers seem to catch up," Swain said. And there's some indication that the brain patterns for stay-at-home dads are more similar to the changes that moms go through. Swain and his colleagues are still trying to figure out exactly how the parenthood effect works on the neurological level — and how moms and dads get to the same place by different hormonal paths.

Rilling said the study of the fatherhood effect is a "wide open frontier."

"Humans are an alloparental species, which means mothers get help," he said. "In some cultures it's the father, but in other cultures it's the grandmother, the aunts, the older children. Fathers seem to be particularly important in modern developed Western nations like the U.S., because there are so many people who are living in isolated nuclear families, largely separated from their extended family. That limits the number of potential helpers out there. ... It's really important that fathers step up."